The present invention relates, in general, to mold assemblies, and more particularly, to a method and apparatus for forming a runner in a mold assembly.
Workpieces, such as semiconductor devices, are encapsulated to protect them from becoming damaged by mechanical stresses, moisture, or the like. In parting plane mold assemblies, encapsulation is accomplished by placing a workpiece in a first section of a mold assembly and mating the first section with a second section, wherein the step of mating encloses the workpiece in a mold cavity and forms a plurality of runners extending from the mold cavity. Subsequently, an encapsulation material, commonly referred to as a molding compound, is injected through the plurality of runners into the mold cavity, thereby encapsulating the workpiece.
Although this technique has been useful for encapsulating workpieces, it requires machining the mold assembly sections and forming the workpieces to exacting tolerances. A particularly critical problem in parting plane mold assemblies is manufacturing the mold sections so that there are no spaces or gaps along the mating surfaces. Spaces or gaps prevent proper sealing between the mold sections and allow seepage of the encapsulation material from runners formed when the mold sections are mated, i.e., the formation of flash. In addition, workpieces whose dimensional variations exceed specified limits may also cause flash formation by preventing the proper sealing of the runners. Since additional processing steps are required to remove the flash, dimensional variations of either the mold assembly or the workpieces increases the costs associated with encapsulating workpieces.
Accordingly, it would be advantageous to have a method and apparatus for forming a runner in a mold assembly that eliminates flash formation. The method and apparatus should create a seal between two mold sections by imparting a uniform pressure along a parting plane formed by mating the two mold sections. It would be of further advantage for the method and apparatus to prevent flash formation by compensating for dimensional variations of the components of the mold assembly or the workpieces.